In lambda closed-loop control methods with precontrol, values of operating variables are detected and a particular precontrol value is supplied in dependence upon the detected values which determines the quantity of fuel which is supplied to a particular cylinder air charge. As a rule, these precontrol values are base injection times. These precontrol values are determined for pregiven conditions such as for the operation of an engine at 20.degree. C. and 9.8.times.10.sup.4 Pa (a technical atmosphere). In practical operation, if the engine is operated under conditions different from those under which the precontrol values were determined, then the particular desired lambda value is not precisely obtained with the precontrol values.
Deviations are compensated with the aid of the lambda control and the adaptation. Adaptation methods function generally in that they integrate the values of the manipulated variable of the lambda controller and multiplicatively or additively couple the integrated value with the precontrol values or with the precontrol values corrected with the control variable. The nature of the coupling and the coupling position during the course of the signal is dependent upon the nature of the change to be adapted. For the following, only that adaptation component is of interest which considers changes which become effective multiplicatively for the correction of precontrol values.
With this adaptation component, deviations can be adapted in the fuel characteristics from the characteristics of the fuel with which the precontrol values were calibrated. The situation which forms the basis of this adaptation will now be explained.
If an engine is driven with gasoline as the fuel, then specific injection times are required to obtain a particular pregiven lambda value for specific operating conditions. If the same engine is driven with methanol in lieu of gasoline, then the same operating conditions require approximately twice the injection times in order to again obtain the same lambda values. In this connection, it is noted that the multiplicative difference between the injection times is somewhat greater than 2; however, for simplicity, the following will be premised on a factor of 2. It is also noted that it is hardly possible to operate an engine selectively in all operating conditions with only gasoline or with only methanol. In the practical application, the limit of the operability of a gasoline engine for all conventional operating conditions lies at a mixture of approximately 85% methanol with 15% gasoline. Such a mixture is noted in the following as a M85-mixture. Again for the sake of explanation, it will however be assumed almost exclusively that either the tank is filled only with gasoline or only methanol.
If the calibration of the base injection times (precontrol values) took place using a M50-mixture, then the base injection times must be multiplied by the factor 1/1.5 (if in the practical operation, gasoline is used) in order to arrive at the desired lambda values without closed-loop control. If the engine is driven with methanol, then the base injection times must in contrast be multiplied by the factor 2/1.5.
These factors adjust for lambda-controlled operation of an internal combustion engine because of an adaptation. The adaptation takes place however relatively slowly to avoid control oscillations because of the feedback operation of the control. The conventional adaptation speeds are so selected that deviations such as caused by greater air pressure changes or temperature changes can be adapted within several minutes. Such changes can amount to approximately 15% if, for example, a vehicle is switched off when the air is warm and the air pressure is low and is then started again a few days later when the air is significantly cooler and the pressure is high. Deviations such as those caused by changed fuel characteristics can however be substantially greater. For example, if a vehicle is first driven with gasoline and then the tank which is almost empty is filled with M85-mixture, the injection times must be increased in length by 85%. The adaptation operation can then have a duration in the order of magnitude of approximately 10 minutes.
Such long adaptation times can lead to considerable difficulties in practical operation. As a first example, assume a so-called interstate highway stop during which a driver tanks the vehicle at an interstate service station where the fuel deviates considerably (for example in its methanol content) from the fuel which was first in the tank. If, after filling the tank and the driver drives at full load, this leads to the consequence that in conventional lambda control methods used at the present time, no control and therefore no adaptation takes place. This is the case because at full load, uncontrolled precontrol operation takes place with the purpose of providing a rich mixture. The fact that in the mentioned operating case, no adaptation takes place for the changed fuel characteristics, has the consequence that the base injection times are continuously multiplied by the incorrect adaptation factor corresponding to the earlier fuel whereby injection times are continuously adjusted which deviate considerably from the actual desired times.
A still more critical case is that which is here described as the motel stop. In this case, a driver tanks his vehicle before stopping at the motel, drives the vehicle into the motel garage and then starts the vehicle again only after spending the night. When starting, and especially in the subsequent warm-up, the problem is presented that in this operating condition also, as at full load, no lambda control takes place. The mixture for the warm-up is therefore determined on the basis of a completely incorrect adaptation factor. This has as a consequence that the vehicle hardly develops power and under some circumstances, the engine can cease operating soon after the starting operation. If the engine does continue to operate and finally develops power which is somewhat usable, the driver will be tempted to soon again drive at full load. The above-mentioned problem then follows the warm-up problem. A condition similar to that of the motel stop can develop when the driver, who while driving has filled the tank with a dissimilar fuel, then again tanks the vehicle shortly before returning home with the fuel customarily used by the driver. Until the driver has then driven the short remaining distance, the adaptation factor has not yet adapted to the new fuel characteristics. With a later start with a cold engine, similar problems can develop as in the situation of the motel stop.
In order to avoid the problems described above, it is known to place a methanol sensor in the tank to measure the methanol concentration of the fuel mixture in the tank. The corrective factor is adjusted in dependence upon the measured concentration. If the corrective factor is, for example, 1.0 for gasoline and then a M85-mixture is added to the almost empty tank, the sensor detects a methanol concentration of 85% which has the consequence that a corrective factor of 1.85 is immediately adjusted. All problems can be avoided by means of this function which are concerned with that the fuel mixture with which an engine is driven can suddenly change by filling the tank.
The advantage just mentioned above which is conditioned upon the use of a methanol sensor also presents the considerable disadvantage that such sensors are very expensive when they are to operate with the required accuracy.